DE102018205778A1 - Interferometer and method of making an interferometer - Google Patents

Abstract

The invention relates to an interferometer (100) consisting of a holding element (106) with an actuation recess (108), of a first mirror element (102), which is arranged opposite the holding element (106) of the Aktuierungsausnehmung (108) or can be arranged, and from a second mirror element (104), which is arranged opposite the first mirror element (102) in a mirror spacing or can be arranged to form an optical gap (112), wherein the first mirror element (102) between the second mirror element (104) and the retaining element ( 106) is arranged or can be arranged and the optical gap (112) is spatially separated from the actuation recess (108) by the first mirror element (102). Furthermore, the interferometer (100) comprises a pair of electrodes comprising a first actuation electrode (114) which is defined, arranged and / or arrangeable on or in one of the mirror elements (102) or (104) and a second actuation electrode (116), which is defined on a side of the Aktuierungsausnehmung (108) opposite the first actuation electrode (114), arranged and / or can be arranged. The mirror spacing can be changed by applying an electrical voltage to the electrode pair.

Description

State of the art

The invention is based on a device or a method according to the preamble of the independent claims. The subject of the present invention is also a computer program.

Tunable spectral filters with the possibility of miniaturization can be realized by means of MEMS technology, for example as a Fabry-Pérot interferometer. It is exploited that a cavity consisting of two plane-parallel, highly reflective mirrors with a mutual distance in the range of optical wavelengths shows a strong transmission only for wavelengths in which the cavity length corresponds to an integer multiple of half the wavelength. The distance, also called cavity length, can be changed for example by means of electrostatic or piezoelectric actuation, whereby a spectrally tunable filter element is formed. A critical influencing factor on the performance of such a spectrometer is the parallelism or plane parallelism of the two mirrors. This should be as high as possible, so that a defined cavity length with the highest possible effective finesse arises between the two mirrors.

Much of the known Fabry-Perot interferometer uses an electrostatic actuation of the mirrors, as shown for example in the publications US 2012050751 . US 2014022643 or DE10226305 is described. In this case, a voltage is applied between two electrodes, which are located at the level of the two mirrors or, alternatively, on a plane parallel to the mirrors, so that due to the electrostatic attraction, both mirrors move towards each other. The mirrors are designed either as membrane mirrors or massive mirrors on a substrate.

In this form of electrostatic actuation, the maximum travel of the mirrors relative to one another is physically limited to one third of the initial spacing of the electrodes, since spontaneous snap-together of the electrodes occurs at larger travel distances, also called snap-in or pull-in, which causes the Fabry -Perot interferometer can be damaged. For a Fabry-Perot interferometer, the largest possible travel path is advantageous because this results in the tunable wavelength range, which can be addressed interferometrically. The travel can be increased, for example, by attaching electrodes and mirror layer to at least one of the mirrors at different levels. In Scripture US2014022643 For example, the mirror is mounted on a pedestal and the electrodes are mounted in a recess so that the distance of the electrodes is greater than the distance of the mirror. However, the mirrors continue to be actuated towards each other. Due to the larger electrode spacing, a higher actuation voltage is required for the same travel path. Furthermore, such an approach with juxtaposed and different sized columns for optics and actuation is difficult to realize for membrane levels.

In the DE 69628669 T2 For example, a tunable Fabry-Perot interferometer is described that includes two diaphragms with stoppers disposed on two opposite sides of a support structure. Each membrane is connected to a voltage source. By applying an electrical voltage, the membranes can each be deflected relative to the support structure and thus relative to each other.

In summary, the prior art thus allows only a device architecture in which the maximum transmission wavelength due to the initial mirror spacing at rest is fixed by the process control and the minimum transmission wavelength is limited by the electrostatic snap together.

Disclosure of the invention

Against this background, with the approach presented here, an interferometer, a method for producing an interferometer, furthermore a device which uses this method, and finally a corresponding computer program according to the main claims are presented. The measures listed in the dependent claims advantageous refinements and improvements of the independent claim device are possible.

The approach presented here is based on the finding that an interferometer, such as a tunable Fabry-Perot interferometer with electrostatic actuation for reducing or increasing a mirror spacing, can be realized with mirrors arranged on a common side of a substrate or between a cap and the substrate are arranged. Advantageously, a separation of an optical gap between the mirrors of an actuation gap makes it possible to expand the maximum travel independently of the transmission range of the interferometer. This overcomes the lack of typically limited tunability of Fabry-Perot interferometers due to insufficient electrostatic actuation travel. Thus, an independent adjustment of an initial pass wavelength and a Traversing possible. For example, the approach presented here also allows bidirectional actuation of the mirrors, so that at a prescribed maximum voltage the travel is increased compared to a unidirectional actuation. Furthermore, the risk of electrostatic snap-together or contraction of the mirror layers is reduced since, due to the larger possible travel, the system can be operated further from the pull-in point and, in the case of unidirectional operation, the mirrors are actuated away from one another, ie the distance between them Mirroring is increased.

The approach described here allows the creation of mechanically and optically symmetrical systems that are more robust to the coupling of external disturbances such as vibrations. Since the structure also has a high degree of symmetry from an optical point of view with respect to the refractive indices of the materials involved, the maximum transmission is also increased since the mirrors can be produced with identical reflection.

• einem Halteelement mit einer Aktuierungsausnehmung;
• einem ersten Spiegelelement, das am Halteelement der Aktuierungsausnehmung gegenüberliegend angeordnet oder anordenbar ist;
• einem zweiten Spiegelelement, das dem ersten Spiegelelement in einem Spiegelabstand gegenüberliegend angeordnet oder anordenbar ist, um einen optischen Spalt zu bilden, wobei das erste Spiegelelement zwischen dem zweiten Spiegelelement und dem Halteelement angeordnet oder anordenbar ist und der optische Spalt durch das erste Spiegelelement von der Aktuierungsausnehmung räumlich getrennt ist; und
• einem Elektrodenpaar aus einer ersten Aktuierungselektrode, die an oder in einem der Spiegelelemente definiert, angeordnet und/oder anordenbar ist, und einer zweiten Aktuierungselektrode, die an einer der ersten Aktuierungselektrode zugewandten Seite der Aktuierungsausnehmung angeordnet oder anordenbar ist, wobei der Spiegelabstand durch Anlegen einer elektrischen Spannung an das Elektrodenpaar änderbar ist.

An interferometer with the following features is presented:

• a holding element with an Aktuierungsausnehmung;
• a first mirror element, which is arranged opposite to the holding element of Aktuierungsausnehmung or can be arranged;
• a second mirror element disposed or arrangeable opposite to the first mirror element at a mirror spacing to form an optical gap, the first mirror element being disposed or disposable between the second mirror element and the support element and the optical gap being separated from the actuation recess by the first mirror element is spatially separated; and
• a pair of electrodes from a first actuation electrode, which is defined, arranged and / or arrangeable on or in one of the mirror elements, and a second actuation electrode which is arranged or can be arranged on a side of the actuation recess facing the first actuation electrode, wherein the mirror distance is established by applying an electrical actuation electrode Voltage to the electrode pair is changeable.

An interferometer may, for example, be understood to mean a Fabry-Perot interferometer or another device for filtering electromagnetic waves by means of interference. The interferometer can be realized for example in MEMS technology. A first or second mirror element may be understood to mean a reflective element which is permeable to electromagnetic radiation, for example in the form of a mirror membrane or a mirror layer. By way of example, a holding element can be understood as meaning an element which has a substrate layer, for example in the form of a substrate wafer, or a cap of the interferometer, for example in the form of a cap wafer. In addition, the holding element may also have an additional layer such as a sacrificial layer, in which the Aktuierungsausnehmung is introduced. For example, this can be made possible in a selection of the material of the additional layer a particularly simple design of the Aktuierungsausnehmung in the holding element, yet a stable support, for example, the mirror elements and the electrode pair can be ensured. A spatial separation of the optical gap from the Aktuierungsausnehmung by the first mirror element can be understood that, for example, the first mirror element between the optical gap and the Aktuierungsausnehmung is arranged. In this case, the first mirror element does not necessarily have to be designed in such a way that a fluidic separation of the optical gap from the actuation recess is ensured; For example, the first mirror element can also be perforated or have a performance pattern, so that the first mirror element nevertheless spatially separates the optical gap from the actuation recess according to the understanding of this description.

For example, in one embodiment of the approach presented here, an actuation gap can be delimited by the first mirror element, which is arranged opposite to the retaining element of the actuation recess or can be arranged. By the first mirror element, the optical gap can be spatially separated from the Aktuierungsspalt.

Particularly advantageous is an embodiment of the approach presented here, in which the first actuation electrode is defined, arranged and / or can be arranged on or in the first mirror element. Such an embodiment offers the advantage that when an electrical voltage is applied to the electrode pair, the optical gap can be increased, so that the risk of snap-fastening of the two mirror elements during actuation is reduced.

According to one embodiment, the holding element may comprise a substrate or, additionally or alternatively, a cap, wherein the holding element further comprises an additional layer in which the Aktuierungsausnehmung is formed. This allows an Aktuierungsspalt depending on the embodiment be formed by means of a substrate layer or cap. At the same time allows the use of the additional layer, which is applied for example on or on the substrate or the cap, a flexible and technically simple design of Aktuierungsausnehmung in the holding element.

According to a further embodiment, the first mirror element can have at least one stop for preventing a connection, in particular as a result of a striking, of the first mirror element to the actuation recess and / or to the second mirror element. Additionally or alternatively, the second mirror element may have at least one stop for preventing a connection, in particular after a striking of the second mirror element to the first mirror element. In the present case, a connection can be understood to mean, for example, a permanent connection, for example a material-locking connection such as gluing. Under a stop, for example, a stop knuckle or a stop pin are understood. As a result, large-area contact and connection / gluing between the first mirror element and the opposite boundary of the Aktuierungsausnehmung or between the first mirror element and the second mirror element can be prevented.

The interferometer may further comprise an auxiliary electrode pair of a first additional actuation electrode defined on or in the first mirror element or a side of the first mirror element opposite the second mirror element and a second additional actuation electrode located on an opposite side of the second auxiliary actuation electrode Mirror element is arranged or can be arranged, have. In this case, the mirror spacing can be changed by applying an electrical voltage to the additional electrode pair. This allows a bidirectional change in mirror spacing.

Furthermore, it is advantageous if the interferometer has a cap element which is arranged opposite to the second mirror element at a cap spacing or can be arranged to form a (further) Aktuierungsspalt, wherein the (further) Aktuierungsspalt is spatially separated by the second mirror element from the optical gap , In this case, the interferometer, a further pair of electrodes from a first further Aktuierungselektrode which is arranged or can be arranged on a side opposite the cap member side of the second mirror element, and a second further Aktuierungselektrode, which is arranged or can be arranged on one of the first further Aktuierungselektrode opposite side of the cap member exhibit. The cap spacing, which for example designates the distance of the second mirror element from the cap, can be changed by applying an electrical voltage to the further actuation electrode pair. As a result, the maximum adjustable distance of the mirror elements can be further increased. In particular, the cap member may have an antireflection coating and thereby have a lower reflectance and / or higher transmittance than an uncoated cap member of the same material.

According to one embodiment, the cap element can be embodied as a transparent substrate, in particular a glass wafer (borosilicate glass or alkali-free glass or soda-lime glass) or quartz wafer. As a result, a particularly broadband interferometer can be realized, which is also suitable for wavelengths below 1100 nm, more precisely, wavelengths which correspond to a photon energy above the band gap of the substrate material.

According to a further embodiment, the interferometer may comprise at least one additional cap wafer, which may be arranged on a side of the holding element opposite the cap element in order to cap the interferometer on both sides. As a result, the interferometer can be protected against interference or environmental influences.

It is advantageous if the additional cap wafer is designed as a glass wafer and / or quartz wafer, for example, similar to the aforementioned cap member. Also by this embodiment, a particularly broadband interferometer can be realized.

According to a further embodiment, the interferometer may have at least one measuring electrode pair for measuring the mirror spacing. The measuring electrode pair may, for example, have at least two measurement electrodes arranged opposite one another. Depending on the embodiment, the measuring electrodes may be the actuation electrodes or additional electrodes. As a result, the mirror distance can be measured precisely (for example, also indirectly), but at least the change thereof by means of an indirect measurement.

According to a further embodiment, the holding element may have a through opening opposite the first mirror element. As a result, the interferometer can be operated in a spectral range in which the holding element is absorbent. Furthermore, it can reduce reflections in the optical path of the interferometer.

• Anordnen eines ersten Spiegelelements an einem Halteelement gegenüber einer Aktuierungsausnehmung des Halteelements, und eines zweiten Spiegelelements in einem Spiegelabstand gegenüber dem ersten Spiegelelement, um einen optischen Spalt zu bilden, wobei das erste Spiegelelement zwischen dem zweiten Spiegelelement und dem Halteelement angeordnet wird und der optische Spalt durch das erste Spiegelelement von der Aktuierungsausnehmung räumlich getrennt ist; und
• Bilden eines Elektrodenpaars durch Anordnen oder Definieren einer ersten Aktuierungselektrode an oder in einer der Aktuierungsausnehmung zugewandten Seite eines der Spiegelelemente und einer zweiten Aktuierungselektrode an einer der ersten Aktuierungselektrode gegenüberliegenden Seite der Aktuierungsausnehmung, wobei der Spiegelabstand durch Anlegen einer elektrischen Spannung an das Aktuierungselektrodenpaar änderbar ist.

The approach presented here also provides a method for producing an interferometer, the method comprising the following steps:

• Arranging a first mirror element on a holding element relative to an actuation recess of the holding element, and a second mirror element in a mirror spacing with respect to the first mirror element to form an optical gap, wherein the first mirror element is arranged between the second mirror element and the holding element and the optical gap passes through the first mirror element is spatially separated from the actuation recess; and
• Forming a pair of electrodes by arranging or defining a first Aktuierungselektrode on or in the Aktuierungsausnehmung facing side of the mirror elements and a second Aktuierungselektrode on one of the first Aktuierungselektrode opposite side of the Aktuierungsausnehmung, wherein the mirror spacing is changeable by applying an electrical voltage to the Aktuierungselektrodenpaar.

This method can be implemented, for example, in software or hardware or in a mixed form of software and hardware, for example in a control unit.

The approach presented here also creates a device that is designed to perform the steps of a variant of a method presented here in appropriate facilities to drive or implement. Also by this embodiment of the invention in the form of a device, the object underlying the invention can be solved quickly and efficiently.

For this purpose, the device may comprise at least one computing unit for processing signals or data, at least one memory unit for storing signals or data, at least one interface to a sensor or an actuator for reading sensor signals from the sensor or for outputting data or control signals to the sensor Actuator and / or at least one communication interface for reading or outputting data embedded in a communication protocol. The arithmetic unit may be, for example, a signal processor, a microcontroller or the like, wherein the memory unit may be a flash memory, an EPROM or a magnetic memory unit. The communication interface can be designed to read or output data wirelessly and / or by line, wherein a communication interface that can read or output line-bound data, for example, electrically or optically read this data from a corresponding data transmission line or output in a corresponding data transmission line.

In the present case, a device can be understood as meaning an electrical device which processes sensor signals and outputs control and / or data signals in dependence thereon. The device may have an interface, which may be formed in hardware and / or software. In the case of a hardware-based embodiment, the interfaces can be part of a so-called system ASIC, for example, which contains a wide variety of functions of the device. However, it is also possible that the interfaces are their own integrated circuits or at least partially consist of discrete components. In a software training, the interfaces may be software modules that are present, for example, on a microcontroller in addition to other software modules.

Also of advantage is a computer program product or computer program with program code which can be stored on a machine-readable carrier or storage medium such as a semiconductor memory, a hard disk memory or an optical memory and for carrying out, implementing and / or controlling the steps of the method according to one of the embodiments described above is used, especially when the program product or program is executed on a computer or a device.

• 1 eine schematische Darstellung eines Interferometers gemäß einem Ausführungsbeispiel;
• 2 eine schematische Darstellung eines Interferometers gemäß einem Ausführungsbeispiel;
• 3 eine schematische Darstellung eines Interferometers gemäß einem Ausführungsbeispiel;
• 4 eine schematische Darstellung eines Interferometers gemäß einem Ausführungsbeispiel;
• 5 eine schematische Darstellung eines Interferometers gemäß einem Ausführungsbeispiel;
• 6 eine schematische Darstellung eines Interferometers gemäß einem Ausführungsbeispiel;
• 7 eine schematische Darstellung eines Interferometers gemäß einem Ausführungsbeispiel;
• 8 eine schematische Darstellung eines Interferometers gemäß einem Ausführungsbeispiel;
• 9 ein Ablaufdiagramm eines Verfahrens gemäß einem Ausführungsbeispiel;
• 10 ein Blockschaltbild einer Vorrichtung gemäß einem Ausführungsbeispiel; und
• 11 eine schematische Darstellung eines Interferometers gemäß einem Ausführungsbeispiel.

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description. It shows:

• 1 a schematic representation of an interferometer according to an embodiment;
• 2 a schematic representation of an interferometer according to an embodiment;
• 3 a schematic representation of an interferometer according to an embodiment;
• 4 a schematic representation of an interferometer according to an embodiment;
• 5 a schematic representation of an interferometer according to an embodiment;
• 6 a schematic representation of an interferometer according to an embodiment;
• 7 a schematic representation of an interferometer according to an embodiment;
• 8th a schematic representation of an interferometer according to an embodiment;
• 9 a flowchart of a method according to an embodiment;
• 10 a block diagram of a device according to an embodiment; and
• 11 a schematic representation of an interferometer according to an embodiment.

In the following description of favorable embodiments of the present invention, the same or similar reference numerals are used for the elements shown in the various figures and similar acting, with a repeated description of these elements is omitted.

1 shows a schematic representation of an interferometer 100 according to an embodiment. The interferometer 100 comprises a first mirror element 102 and one opposite to the first mirror element 102 arranged in a changeable mirror distance second mirror element 104 , The first mirror element 102 , here by way of example a lower mirror, is on a holding element 106 , which is based here by way of example on a substrate (wafer) 107 and an additional layer described in more detail below, with respect to an actuation recess 108 of the holding element 106 arranged and limited with this one Aktuierungsspalt 110 , By way of example, a cross section through a Fabry-Perot interferometer is shown, in which the actuation via the Aktuierungsspalt 110 electrostatically, with a starting distance of the Aktuierungsspaltes 110 independent of the mirror distance is adjustable. The second mirror element 104 , here by way of example an upper mirror, limited together with the first mirror element 102 an optical gap 112 which corresponds to the mirror spacing, wherein the first mirror element 102 between the holding element 106 and the second mirror element 104 is arranged, ie, the two mirror elements 102 . 104 are on a common side of the retaining element 106 arranged. The optical gap 112 is through the first mirror element 102 from the Aktuierungsspalt 110 or the Aktuierungsausnehmung 108 spatially separated.

To change the mirror distance, the interferometer points 100 a pair of electrodes from a first actuation electrode 114 and a second actuation electrode 116 on. The first actuation electrode 114 is at one of the Aktuierungsausnehmung 108 facing side of the first mirror element 102 arranged while the second actuation electrode 116 at one of the first actuation electrode 114 opposite side of Aktuierungsausnehmung 108 is arranged. Advantageously, one or more of the actuation electrodes 114 . 116 be executed annular. The mirror spacing can be changed by applying a suitable actuation voltage to the electrode pair. Likewise conceivable is an integration of the first actuation electrode 114 in the first mirror element 102 itself (ie a definition of the first actuation electrode 114 in the first mirror element 102 ), for example by a local doping. In one embodiment, this first Aktuierungselektrode 114 be performed flat in the simplest case and result in the Aktuation by, for example, annular counter electrode.

Marked in the 1 also optional stop knobs 118 (which may also be referred to as a stopper), an optional antireflection coating 120 on both sides of the retaining element 106 , more specifically, the substrate 107 , in the region of an optical aperture, a layer reflecting and / or absorbing in the relevant wavelength range as the aperture stop 122 to form an optical aperture 122 ' , a first sacrificial layer 124 (as an additional layer for forming the Aktuierungsausnehmung herein) and a second sacrificial layer 126 (As another additional layer for holding the second mirror element 104 and for the formation of the optical gap 112 here one). For example, the first sacrificial layer 124 and / or the second sacrificial layer 126 , which is used here as an additional layer or additional layer, particularly favorable etching properties with respect to the substrate 107 and have the mirror elements, so that the Aktuierungsausnehmung 108 particularly easy to train in this additional layer. Here is the first mirror element 102 between the first sacrificial layer 124 and the second sacrificial layer 126 arranged while the second mirror element 104 on the second sacrificial layer 126 is arranged, wherein the second sacrificial layer 126 between the first mirror element 102 and the second mirror element 104 is arranged. By way of example, the electrical contacting of the electrode pair is effected by means of a contact element 128 that at least through the second sacrificial layer 126 passed through. This is a simplified representation showing the case of unidirectional actuation.

In one embodiment, the Fabry-Perot interferometer comprises 100 two mutually spaced mirror elements 102 . 104 resting on a retaining element 106 on the basis of a substrate 107 are supported, wherein at least one of the mirror elements is movable, the movement is induced by means of actuation, the actuation is electrostatic, by the actuation of the mirror distance is variable and the variation involves an active enlargement of the mirror spacing.

Depending on the embodiment, the mirror elements 102 . 104 implemented as DBR membrane mirror (DBR = distributed Bragg reflector) or as a mechanical support membrane with mirror layer. The membrane mirrors preferably have a tensile bias. The spacing of both the mirror elements to one another and the mirror elements to the substrate takes place approximately by means of a surface micromechanical sacrificial layer, such as, for example, the first sacrificial layer 124 and / or the second sacrificial layer 126 , The sacrificial layer is, for example, by means of an etching process at least in the optically free region of the optical aperture 122 ' removed so that the mirror elements 102 . 104 are released. On at least one membrane and the substrate 107 or at least one membrane and / or a cap described in more detail below, an electrode is arranged which, depending on the exemplary embodiment, has a locally doped semiconductor material or is metallic. The mirror elements 102 . 104 For example, as Bragg mirror of dielectric or layers which have no or low absorption in a wavelength range, for example Si, Ge, SiN, SiO ₂ , SiCN, TiO _x or SiC or as a support membrane with metal mirror (with protective layers of a the preceding materials), such as Ag, Cu, Al or Au. Active position detection can be carried out, for example, by means of capacitive or piezoresistive detection. The MEMS wafer is optionally capped on one or both sides, such as with a silicon wafer or glass wafer cap.

A fundamental problem of electrostatic spring-actuator systems is the so-called pull-in phenomenon, in which a stable distance can no longer be set between two actuation electrodes as soon as a critical travel distance is exceeded. The reason for this is the over-linear increase in the electrostatic force with the travel compared to the linear increase of the spring force, which acts as a restoring force. The force exerted by an electrostatic actuator is given by $$F_{el}=-\frac{1}{2}\frac{dC}{dx}{U}^2,$$

wobei d₀ den Spaltabstand des elektrostatischen Aktors in Ruhelage und A dessen Fläche bezeichnet. Als vereinfachte Geometrie wird hierbei ein Plattenkondensator angenommen, der an einer Feder aufgehängt ist. Diese Näherung kann in den meisten Anwendungen auch getroffen werden, da sich viele reale Systeme auf dieses idealisierte Grundsystem zurückführen lassen.Thus, the force of the change of the capacitance C with the travel x and the applied voltage U depends. For a plate capacitor, the capacity applies to: $$C\left(x\right)={\epsilon }_0\frac{A}{d_0-x\text{'}}$$

where d ₀ denotes the gap distance of the electrostatic actuator in the rest position and A its surface. As a simplified geometry in this case a plate capacitor is assumed, which is suspended on a spring. This approximation can also be met in most applications because many real systems can be traced back to this idealized basic system.

bei einem Verfahrweg x. Da eine Auslenkung des Aktors den elektrostatischen Spalt verkleinert, nimmt ebenfalls die elektrostatische Kraft mit steigender Auslenkung zu, sogar überlinear. Daher gibt es eine kritische Auslenkung x_PI, bei der die Federkraft die elektrostatische Kraft nicht mehr kompensieren kann und bei der es keine stabile Gleichgewichtslage gibt. Dieser Punkt wird im Allgemeinen als Pull-in- oder Snap-in-Punkt bezeichnet und liegt unabhängig von der Federhärte k und der Elektrodenfläche immer bei einem Drittel des Spaltabstands d₀/3 und tritt bei einer Spannung U_PI auf, die für das vereinfachte System durch folgenden Zusammenhang gegeben ist: $$U_{PI}=\sqrt{\frac{8}{27}\frac{k{d}_0^3}{{\epsilon }_{0}A}}.$$

Furthermore, this simplified model system is merely illustrative of the underlying problem. For a spring of the spring hardness k then results in a restoring force of $$F_F=-kx$$

at a travel x. Since a deflection of the actuator reduces the electrostatic gap, the electrostatic force also increases with increasing deflection, even over-linear. Therefore, there is a critical displacement x _PI , where the spring force can no longer compensate for the electrostatic force and where there is no stable equilibrium position. This point is generally referred to as pull-in or called snap-in point and is independent of the spring stiffness k and the electrode surface is always in one-third of the gap distance d _0/3 and occurs at a voltage U _PI, the simplified for the System is given by the following relationship: $$U_{PI}=\sqrt{\frac{\mathrm{8th}}{27}\frac{k{d}_0^3}{{\epsilon }_{0}A}},$$

Here, the variables denote the sizes already introduced. This phenomenon represents a fundamental limitation of the electrostatically actuable travel path. In order to realize a Fabry-Perot interferometer with the widest possible spectral working range, it is necessary to find bypass solutions, since inter alia the maximum travel of the mirrors to one another also determines the maximum tunable spectral range , The problem with previously realized Fabry-Perot interferometers can be seen in the fact that the optical gap and the Aktuationsspalt match, which limits the tunability due to the pull-in. The approach presented here circumvents this problem by the spatial separation of exactly these two columns.

According to one embodiment, the mirror elements 102 . 104 designed as tensile strained membranes. These are clamped over a free surface and define in this way the necessary spring. The tensile bias is advantageous in order to reduce gravitational slack of the membranes and to increase the natural frequency of the spring system, whereby a higher robustness against the coupling of external vibrations exists. As material for the membrane mirrors, it is possible to use both dielectric or non-absorbing layer systems, so-called Bragg reflectors, and metallic mirrors on carrier structures or nanostructured plasmonic mirror structures. The mechanical advantage of these systems is a high natural frequency due to the low mass and, associated therewith, a very low positional sensitivity of the membrane mirrors compared to mirror layers, which are applied to solid support structures. At the same time the high natural frequency is achieved at a relatively low spring stiffness, which is also adjustable by a defined layer stress. Another advantage of the approach described here compared to other membrane-based Fabry-Perot interferometers is the realization of a mechanically symmetric system of two mechanically identical mirrors, which has an additional insensitivity to external disturbances such as vibrations. Furthermore, the design of the Fabry-Perot interferometer allows 100 with two movable membranes in addition a bidirectional Aktuation, for example by attraction of the two membranes by the choice of suitable potentials. The prerequisite for this is the definition of suitable electrodes on or in the mirror layers. Such Aktuation is advantageous because it allows an increased maximum adjustable distance of the mirror elements from each other at the same maximum voltage.

For the operation of a Fabry-Pérot interferometer, it is also advantageous if both mirror elements 102 . 104 have the same spectral reflection characteristic. Otherwise, maximum transmission at the pass wavelength can not be achieved, which translates into signal loss. For a Fabry-Pérot interferometer consisting of a membrane mirror and a mirror on a solid substrate, this is difficult to achieve over a wide spectral range, since the two mirrors are not identical, because the membrane mirror is bounded on both sides by air and the substrate mirror becomes bounded on one side by the substrate. According to the approach presented here, both mirror elements automatically become 102 . 104 designed as a membrane, so that they can be made very similar and thus have the same reflection characteristic.

The two reflective membranes have a defined distance from one another and are connected to a substrate material. The reflectivity of the membranes is realized for example by metallic layers on support structures or a Bragg mirror. The membranes have been made movable by suitable sacrificial layer processes in a defined area in a direction perpendicular to the plane of the membranes. It is advantageous to have the highest possible parallelism or plane parallelism of the membranes, which can be achieved, for example, by means of suitable tensile mechanical prestressing of the membrane layers. At least one of the two membranes can be moved by applying an electrical voltage, whereby the distance between the two reflective membranes or mirrors is changed. Thereby, the transmission wavelength of the Fabry-Perot interferometer thus formed can be controlled. To realize this movement, actuation electrodes are located on at least one of the membranes 114 , There are also additional matching actuation electrodes 116 in at least one level above or below the two mirror membranes 102 . 104 , The production of actuation electrodes 114 . 116 takes place, for example, by deposition and possible structuring of a conductive layer or by targeted doping of a semiconductor material.

In 1 By way of example, an embodiment is shown in which Aktuierungselektroden on the lower mirror 102 and on the substrate wafer 107 are located. Between both levels a Aktuierungsausnehmung becomes by a suitable sacrificial layer process 108 realized. By applying a voltage between the pair of electrodes 114 . 116 becomes the first mirror element 102 to the holding element 106 or substrate 107 attracted, causing the distance between the two mirror elements 102 . 104 enlarged from each other. The decisive advantage of this design is that the initial distance of the electrode pairs 114 . 116 largely independent of the initial distance of the two mirror membranes 102 . 104 is adjustable and the travel is not limited to a third of the initial optical mirror spacing by pull-in with sufficiently large electrode spacing.

Optional are as a stop 118 acting knobs are added which increase the deflection of the membranes 102 . 104 limit and contribute to the fact that the structures are easier to separate again from each other, since the contact surface is kept small. Also advantageous are antireflection coatings 120 on all interfaces in the optical path, not part of the mirror 102 . 104 are. Another advantage is the use of the optical aperture 122 ' to limit the optical light path and the angle of incidence by reflection and / or absorption.

2 shows a schematic representation of an interferometer 100 according to an embodiment. In contrast to 1 has the interferometer 100 According to this embodiment, an additional electrode pair of a first Zusatzaktuierungselektrode 200 and a second Zusatzaktuierungselektrode 202 on. In this case, the first additional Aktuierungselektrode 200 at a second mirror element 104 opposite side of the first mirror element 102 arranged while the second Zusatzaktuierungselektrode 202 at one of the first additional actuator electrode 200 opposite side of the second mirror element 104 is arranged. Thus, in addition, an actuation between the mirror elements 102 . 104 possible. The representation of the electrical contact is shown here greatly simplified. For example, no different contacts for the different electrodes are shown in the figure. Also conceivable is an embodiment in which the electrodes 114 and 200 are identical, which is typically the case when realized via the doping.

In other words, it shows 2 an embodiment in which also between the mirror elements 102 . 104 electrodes 200 . 202 are located. As a result, a reduction of the initial mirror spacing is possible in addition. In addition, with the same travel of the mirror elements 102 . 104 reduces the necessary electrical voltages.

3 shows a schematic representation of an interferometer 100 according to an embodiment. In contrast to 2 has the interferometer 100 according to this embodiment in the region of Aktuierungsausnehmung 108 a passage opening 300 in the substrate 107 on. For this example, substrate material was removed in the optical path. This is the Fabry-Perot interferometer 100 also operable in a spectral range in which the substrate 107 is absorbent. In addition, the total transmission of the Fabry-Pérot interferometer 100 is increased by the elimination of reflections at other interfaces in the optical path.

4 shows a schematic representation of an interferometer 100 according to an embodiment. The interferometer 100 essentially corresponds to the above with reference to the 1 to 3 described interferometer. Shown is an embodiment in which an actuation between the first mirror element 102 and an additional cap wafer or cap 400 as the base of the holding element 106 is possible. Here is the first mirror element 102 between the cap wafer and the cap 400 and a substrate (wafer) 107 including the first sacrificial layer 124 arranged on which the second mirror element 104 is applied. Also marked are an optional wafer bonding connection 402 (Here, for example, as an additional layer of the cap 400 to form the retaining element 106 ) and an optional cap antireflection coating 404 on one of the actuation gap 110 facing away and / or the facing side of the cap wafer or the cap 400 is applied.

In addition to an actuation between a first mirror element and holding element or substrate, an implementation is also conceivable in which the electrodes are mounted on an additional cap wafer or the cap 400 allow a movement of the upper mirror upwards, which also increases the mirror spacing. As in 4 Recognizable, the distance of the electrode pair is here through the wafer bonding compound 402 set. Through the cap 400 In addition, the inclusion of a defined pressure is possible. In addition, the cap protects 400 of the holding element 106 the Fabry-Perot interferometer 100 from external influences such as moisture and impurities. The cap wafer or the cap 400 is made of silicon or glass, for example.

5 shows a schematic representation of an interferometer 100 according to an embodiment. The interferometer 100 includes according to this embodiment, a cap member 500 Similar to the above with reference to 4 described cap wafer 400 , The cap element 500 is via a wafer bonding connection 501 the second mirror element 104 arranged at a cap distance opposite and limited with this another Aktuierungsspalt 502 passing through the second mirror element 104 from the optical gap 112 is spatially separated. In addition, the interferometer includes 100 another pair of electrodes from a first further Aktuierungselektrode 504 attached to the cap element 500 opposite side of the second mirror element 104 is arranged, and a second further Aktuierungselektrode 506 attached to one of the first further actuation electrodes 504 opposite side of the cap member 500 is arranged. Again, an embodiment is conceivable in which the integration of the electrode or thereby a coincidence of two electrodes is possible, as would be the case for example by doping. The cap spacing can be changed by applying a suitable actuation voltage to the further electrode pair. The two mirror elements 102 . 104 are thus movable by applying the appropriate electrical voltages in both directions. The required for comparable changes in the mirror spacing electrical voltages can be further reduced here due to the bidirectional actuation.

6 shows a schematic representation of an interferometer 100 according to an embodiment. In contrast to 5 has the interferometer 100 according to this embodiment, the through hole 300 out 3 as well as one additional cap wafer 600 on top of the passage opening 300 on a first mirror element 102 opposite side of the holding element 106 or substrate 107 covering, so that the passage opening 300 on the one hand from the first mirror element 102 on the other hand, from the additional cap wafer 600 is covered. For example, the additional cap wafer 600 by means of an additional wafer bonding connection 602 on the holding element 106 appropriate.

When the top cap wafer 500 in combination with the through hole 300 is used in the substrate material, the additional connection with the second cap wafer 600 be useful on the back to continue encapsulating the Fabry-Pérot interferometer 100 and to achieve a stabilization of the component. Again, on the cap wafers 500 . 600 optional antireflection layers 404 and applied optical apertures.

7 shows a schematic representation of an interferometer 100 according to an embodiment. The interferometer 100 is very similar to the previous one with reference to 2 described embodiment. In contrast to 2 has the interferometer 100 according to 7 however, one measuring electrode pair of measuring electrodes 700 on, each in the optical gap 112 are arranged and designed to measure the mirror spacing, for example, capacitive. This is a position control in the interferometer 100 integrated.

Alternatively, the actuation electrodes can be used for this measurement. The capacitive distance measurement is optionally carried out with more than two pairs of electrodes, for example, to realize a differential-capacitive evaluation.

8th shows a schematic representation of an interferometer 100 according to an embodiment. The interferometer 100 is similar to the previous one with reference to 2 constructed interferometer constructed. In contrast, the interferometer points 100 according to 8th however, additional electrodes 800 on that in the optical path of the interferometer 100 are integrated. Exemplary are the additional electrodes 800 Opposite and centered in the optical gap 112 arranged.

The warping of the membranes 102 . 104 in the optical domain is minimized by electrostatic forces in this region through the additional electrodes 800 be prevented with the same electrostatic potential, which is advantageous to the achievable spectral resolution of the Fabry-Perot interferometer 100 effect.

9 shows a flowchart of a manufacturing process 900 according to an embodiment, for example, for the preparation of a foregoing with reference to the 1 to 8th described interferometer can be executed. The procedure 900 includes a step 910 in that the first mirror element is arranged on the holding element with respect to the Aktuierungsausnehmung, and the second mirror element is arranged in the mirror spacing with respect to the first mirror element to form the optical gap. In this case, the first mirror element is arranged between the second mirror element and the retaining element and the optical gap is spatially separated by the first mirror element from the Aktuierungsausnehmung. In a further step 920 The electrode pair is formed by arranging or defining the first actuation electrode on or in a side of one of the mirror elements facing the actuation recess and a second actuation electrode on an opposite side of the actuation recess from the first actuation electrode. The mirror spacing is changeable by applying an electrical voltage to the electrode pair. It should be noted that a manufacturing process includes these steps. However, the order of execution of such steps is not significant.

10 shows a block diagram of a device 1000 for making an interferometer, the device 1000 one unity 1010 for arranging a first mirror element on a holding element in relation to an actuation recess of the holding element, and a second mirror element in a mirror spacing with respect to the first mirror element to form an optical gap, wherein the first mirror element is arranged between the second mirror element and the holding element and the optical mirror Gap is spatially separated by the first mirror element of the Aktuierungsausnehmung. Furthermore, the device comprises 1000 one unity 1020 for forming a pair of electrodes by arranging or defining a first actuation electrode on or in a Aktuierungsausnehmung facing side of the mirror elements and a second Aktuierungselektrode on one of the first Aktuierungselektrode opposite side of the Aktuierungsausnehmung, wherein the mirror spacing can be changed by applying an electrical voltage to the pair of electrodes.

11 shows a schematic representation of an interferometer 100 according to an embodiment. Shown is a particularly favorable embodiment of the above with reference to 6 described interferometer. According to 11 exists the interferometer 100 from a combination of a MEMS Fabry-Perot interferometer 100 with passage opening 300 in the holding element 106 and simultaneously capping with glass wafers as the cap member 500 and the additional cap wafer 600 ,

As a result, comparatively thick silicon areas are removed from the beam path, with transparent capping at the same time, which makes such a structure also suitable for wavelengths below 1100 nm, in which silicon absorbs electromagnetic radiation. By combining with the actuation of the first mirror element 102 , here a lower mirror, down and a broadband mirror so extremely broadband Fabry-Perot interferometer can be realized. Under a broadband Fabry-Perot interferometer can be understood according to the approach presented here, a tunable narrow-band Fabry-Perot interferometer, which is tunable over a wide spectral range.

In addition, the general advantages of a masked Fabry-Perot interferometer are preserved, such as protection from ambient humidity, inclusion of a defined internal pressure, particulate robustness, protection in sawing or size of a chip scale package.

According to this embodiment, the two mirror elements 102 . 104 realized as mirror membranes passing through the optical gap 112 are separated from each other. Here, the first mirror element forms 102 together with the holding element 106 the Aktuierungsspalt 110 ,

Transparent cap materials suitable for wavelengths below 1100 nm include, for example, germanium, quartz, borosilicate glasses, glasses with adapted thermal expansion, other special glasses or even sapphire. The glass wafers are, for example, by eutectic bonding or bonding with seal glass in the interferometer 100 attached.

If an exemplary embodiment comprises a "and / or" link between a first feature and a second feature, then this is to be read so that the embodiment according to one embodiment, both the first feature and the second feature and according to another embodiment either only first feature or only the second feature.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 2012050751 [0003]
• US 2014022643 [0003, 0004]
• DE 10226305 [0003]
• DE 69628669 T2 [0005]

Claims (15)

Interferometer (100) with the following features: a holding element (106) having an actuation recess (108); a first mirror element (102) which is arranged opposite to the holding element (106) of the Aktuierungsausnehmung (108) or can be arranged; a second mirror element (104) disposed or arrangeable opposite the first mirror element (102) at a mirror spacing to form an optical gap (112), the first mirror element (102) being disposed between the second mirror element (104) and the support element (106) is arranged or arrangeable and the optical gap (112) is spatially separated from the actuation recess (108) by the first mirror element (102); and a pair of electrodes comprising a first actuation electrode (114) defined on, arranged and / or arrangeable on or in one of the mirror elements (102, 104) and a second actuation electrode (116) located on an opposite side of the first actuation electrode (114) the Aktuierungsausnehmung (108) is defined, arranged and / or can be arranged, wherein the mirror spacing can be changed by applying an electrical voltage to the electrode pair. Interferometer (100) according to Claim 1 in which the first actuation electrode (114) is defined, arranged and / or arrangeable on or in the first mirror element (102). Interferometer (100) according to one of the preceding claims, wherein the holding element (106) comprises a substrate (107) and / or a cap (400), wherein the holding element (106) further comprises an additional layer (124, 402), in which the Aktuierungsausnehmung (108) is formed. Interferometer (100) according to one of the preceding claims, wherein the first mirror element (102) at least one stop (118) for preventing connection, in particular due to a stop, of the first mirror element (102) to the Aktuierungsausnehmung (108) and / or the second mirror element (104) and / or the second mirror element (104) at least one stop (118) for preventing connection, in particular after striking, the second mirror element (104) to the first mirror element (102). Interferometer (100) according to one of the preceding claims, with an additional electrode pair of a first Zusatzaktuierungselektrode (200), which on or in the first mirror element (102) defined or a second mirror element (104) opposite side of the first mirror element (102) arranged or can be arranged, and a second Zusatzaktuierungselektrode (202) disposed on one of the first Zusatzaktuierungselektrode (200) opposite side of the second mirror element (104) or can be arranged, wherein the mirror spacing by applying an electrical voltage to the additional electrode pair is changeable. Interferometer (100) according to one of the preceding claims, comprising a cap element (500) which is arranged opposite to the second mirror element (104) at a cap distance or can be arranged to form an Aktuierungsspalt (502), wherein the Aktuierungsspalt (502) by the second mirror element (104) is spatially separated from the optical gap (112), and another pair of electrodes comprises a first further actuation electrode (504) defined on or in the first mirror element (102) or an opposite side of the cap element (500) second mirror element (104) is arranged or can be arranged, and a second further Aktuierungselektrode (506), which is arranged or can be arranged on one of the first further Aktuierungselektrode (504) opposite side of the cap member (500), wherein the cap spacing by applying an electrical voltage to the further electrode pair is changeable. Interferometer (100) according to Claim 6 in which the cap element (500) is designed as a transparent substrate, in particular as a glass substrate. Interferometer (100) according to Claim 6 or 7 with at least one additional cap wafer (600) disposed on a side of the support member (106) opposite the cap member (500) for double-capping the interferometer (100). Interferometer (100) according to Claim 8 in which the additional cap wafer (600) is designed as a transparent substrate, in particular as a glass substrate. Interferometer (100) according to one of the preceding claims, comprising at least one measuring electrode pair (700) for measuring the mirror spacing. Interferometer (100) according to one of the preceding claims, wherein the holding element (106) has a first opening (300) opposite the first mirror element (102). A method (900) for making an interferometer (100), the method (900) comprising the steps of: Arranging (910) a first mirror element (102) on a holding element (106) opposite an actuation recess (108) of the holding element (106), and a second mirror element (104) at a mirror distance from the first mirror element (102) about an optical gap (112), wherein the first mirror element (102) is disposed between the second mirror element (104) and the support member (106) and the optical gap (112) is spatially separated from the actuation recess (108) by the first mirror element (102) is; and forming (920) an electrode pair by arranging or defining a first actuation electrode (114) on or in one of the actuation recesses (108) facing side of one of the mirror elements (102, 104) and a second actuation electrode (116) on one of the first actuation electrode (114 ) opposite side of Aktuierungsausnehmung (108), wherein the mirror spacing is changeable by applying an electrical voltage to the electrode pair. Apparatus (1000) comprising units (1010, 1020) adapted to perform the method (900) of Claim 12 execute and / or control. A computer program configured to perform the method (900) in accordance with Claim 12 execute and / or control. Machine-readable storage medium on which the computer program is based Claim 14 is stored.

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